Optical discs such as DVDs (Digital Versatile Discs), BDs (Blu-ray Discs) and the like are used as recording mediums for storing video or data and are required to realize higher density recording.
On such an optical disc, information is recorded by forming recording marks and spaces on a recording layer. In order to perform higher density recording on the optical disc, the recording marks and spaces need to be shortened.
However, as the recording density on the optical disc is increased, the increase of inter-code interference and the deterioration of the SNR (Signal Noise Ratio) become more conspicuous. When the recording marks and spaces are shortened, the amplitude of the reproduction signals is decreased and also there occurs a difference in the amplitude of the reproduction signals in accordance with the combination of the recording marks and spaces by the influence of the inter-code interference. For example, even when recording marks have the same length, the reproduction signals detected especially at edges of the recording marks have different amplitudes due to the difference in the length of the spaces before and after the recording marks. In such a situation, it is considered to be effective to use a PRML (Partial Response Maximum Likelihood) method or the like as the reproduction signal processing method.
In high density recording, as the spaces are shortened, the influence of thermal interference by the recording marks before and after the spaces is increased. Due to the influence of the thermal interference, the position of the edge of the recording mark to be formed is changed. In order to form a recording mark having an appropriate edge position, the pulse waveform of the recording laser light provided in accordance with the length of the space needs to be fine-tuned (recording compensated).
Now, the pulse waveform of the recording laser will be described briefly. FIG. 2 illustrates a recording pulse waveform and a recording power.
FIG. 2(a) shows a cycle Tw of a channel clock, which acts as a reference signal for generating recording data. Based on the cycle Tw, the time interval of the recording marks and spaces of an NRZI (Non Return to Zero Inverting) signal, which is a recording signal shown in FIG. 2(b), is determined. In FIG. 2(b), a recording pattern of 2T mark-2T space-4T mark is shown as an example of a part of the NRZI signal.
FIG. 2(c) shows a multi-pulse train of laser light for forming recording marks. A recording power Pw of the multi-pulse train includes a peak power Pp201 having a heating effect, which is required to form recording marks; a bottom power Pb202 and a cooling power Pc203 both having a cooling effect; and a space power Ps204, which is a recording power in a space. The peak power Pp201, the bottom power Pb202, the cooling power Pc203 and the space power Ps204 are set using an extinction level 205 detected when the laser light is extinguished as a reference level.
The bottom power Pb202 and the cooling power Pc203 are set to an equivalent level of recording power, but the cooling power Pc203 may be occasionally set to a different level from that of the bottom power Pb202 in order to adjust the heat amount at a trailing end of a recording mark. Since no recording mark needs to be formed in a space, the space power Ps204 is generally set to a lower level of recording power (for example, a level equivalent to that of a reproduction power, the bottom power or the like). However, in the case of a rewritable optical disc (for example, a DVD-RAM or a BD-RE), a space needs to be formed by erasing the existing recording mark. Therefore, the space power Ps204 may be occasionally set to a relatively high level of recording power. Also in the case of a write-once optical disc (for example, a DVD-R or a BD-R), the space power Ps204 may be occasionally set to a relatively high level of recording power in order to provide a pre-heating power for forming the next recording mark. Even in such a case, the space power Ps204 is not set to a higher level than that of the peak power Pp201.
The pulse widths are set as follows. A leading pulse width Ttop is set for each of 2T, 3T and 4T or longer recording signals. The pulse widths after Ttop which are present in 3T or longer multi-pulse trains are set to be equal at Tmp. The final pulse width Tmp is set as a last pulse width Tlp. For each recording mark length, a recording start position offset dTtop for adjusting a start position of the recording mark and a recording termination position offset dTs for adjusting an end position of the recording mark are set. The “recording compensation” (space compensation) means changing a recording parameter (for example, dTtop) of a recording pulse in accordance with the length of the space before or after the recording mark.
Laser emission conditions for recording which include the value of each recording power and the value of each pulse width of the multi-pulse trains are described inside the optical disc. Accordingly, as long as the recording powers and the pulse widths of the multi-pulse trains described inside the optical disc can be reproduced and the recording layer of the optical disc can be irradiated with laser light, the recording marks as shown in FIG. 2(d) can be formed.
The recording pulse waveform may be the multi-pulse waveform shown in FIG. 2(c) or any of the waveforms shown in FIG. 3. FIG. 3(a) shows a mono-pulse waveform, FIG. 3(b) shows an L-type pulse waveform, and FIG. 3(c) shows a Castle-type pulse waveform. The recording pulse waveforms are different in the heat amount accumulated in the recording layer of the optical disc, and a recording pulse waveform suitable to the layer characteristics of the recording layer is selected in order to form an optimum recording mark.
An example of recording control method of processing a reproduction signal by the PRML method to perform recording compensation is disclosed in Patent Document No. 1 and Patent Document No. 2.
In Patent Document No. 1, a first bit sequence (state transition matrix having the maximum likelihood), which is a demodulation result, and a second bit sequence (state transition matrix having the second maximum likelihood), which is shifted by 1 bit from the first bit sequence, are used to calculate a Euclidean distance Pa between the reproduction signal and the first bit sequence and a Euclidean distance Pb between the reproduction signal and the second bit sequence, respectively. In addition, an edge shift direction and an edge shift amount of each pattern are detected based on an average value of the calculated |Pa−Pb|−Pstd, where Pstd is the Euclidean distance between the first bit sequence and the second bit sequence. As a result, the adaptive recording parameters, provided in the form of a table based on the lengths of a space and a mark continuous to each other, are optimized in accordance with the edge shift direction and amount corresponding to each pattern.
In Patent Document No. 2, a first pattern (state transition matrix having the maximum likelihood), which is a demodulation result, and a second pattern and a third pattern, which are error patterns with respect to the first pattern are set. Each of the second pattern and the third pattern may be any pattern which becomes an error pattern when the edge to be detected is shifted rightward or leftward with respect to the first pattern. A Euclidean distance E1 between the reproduction signal and the first pattern, a Euclidean distance E2 between the reproduction signal and the second pattern, and a Euclidean distance E3 between the reproduction signal and the third pattern are each calculated. In addition, based on distance difference D2=E2−E1 and distance difference D3=E3−E1, an average value M2 and a standard deviation σ2 of D2 and an average value M3 and a standard deviation σ3 of D3 are found. Then, a correction amount on the recording pulse is determined from the expression Ec=(σ2*M3+σ3*M2)/(σ2+σ3).